Method for treating the etching solution

ABSTRACT

The present etching system includes a processing tank with an etching solution containing silicon, a cooling tank, a pre-heating tank, a first pipe for transferring the etching solution from the processing tank to the cooling tank, a second pipe for transferring the etching solution from the cooling tank to the pre-heating tank, and a third pipe for transferring the etching solution from the pre-heating tank to the processing tank. The present method for treating the etching solution first performs an etching process using the etching solution, which is then cooled to a first temperature to form a silicon-saturated etching solution. After silicon-containing particles larger than a predetermined size are filtered out, the silicon-saturated etching solution is heated to a second temperature to form a non-saturated etching solution for performing another etching process later. The second temperature is preferably at least 10° C. higher than the first temperature.

This is a Continuation of application Ser. No. 11/535,057 filed Sep. 25,2006, which in turn is a Divisional of application Ser. No. 10/943,936.The disclosure of the prior applications is hereby incorporated byreference herein in its entirety.

BACKGROUND

(A) Field of the Invention

The present invention relates to an etching system and method fortreating the etching solution thereof, and more particularly, to anetching system and a method for treating the etching solution with astable selectivity between silicon nitride and silicon oxide.

(B) Description of the Related Art

FIG. 1 to FIG. 3 show a method for fabricating a shallow trenchisolation on a wafer 10 according to the prior art. The shallow trenchisolation is widely used in the fabrication of themetal-oxide-semiconductor (MOS) transistor to form an electricalisolation between transistors. As shown in FIG. 1, the fabrication ofthe shallow trench isolation begins to form an oxide layer 14, a siliconnitride layer 16 and a photoresist layer 18 in sequence on a siliconsubstrate 12, and the pattern of the active region 24 is thentransformed from an active region mask to the photoresist layer 18.

Referring to FIG. 2, a dry etching process is then performed to remove aportion of the silicon nitride layer 16 and the silicon oxide layer 14not covered by the photoresist layer 18 from the silicon substrate 12.The dry etching process continues to the etch silicon substrate 12 toform a shallow trench 20 in the silicon substrate 12.

Refer to FIG. 3, after the photoresist layer 18 is removed, a lineroxide layer 22 is formed on the surface of the shallow trench 20 by athermal oxidation process. Silicon oxide is then deposited in theshallow trench 20 by a chemical vapor deposition (CVD) process, and thesurface of the wafer 10 is planarized by a chemical and mechanicalpolishing (CMP) process. A wet etching process is performed later toremove the silicon nitride layer 16 from the silicon substrate 12 whilepreserving the silicon oxide layer 14 on the surface of the substrate 12and silicon oxide in the shallow trench 20. MOS transistors aresubsequently formed in the active regions 24 on both sides of theshallow trench 20, and the silicon oxide in shallow trench 20 forms theelectrical isolation between MOS transistors.

The conventional method for forming the shallow trench isolation uses aheated phosphoric acid (H₃PO₄) to strip the silicon nitride layer 16.Since subsequent processes to form the MOS transistors are seriouslyinfluenced by both the shape and the cleanness of the surface of thewafer 10, it is very important to control the etching selectivitybetween silicon nitride and silicon oxide. The etching selectivitydepends primarily on parameters such as the etchant, reaction products,reaction temperature, reaction time, etc.; therefore, these parametersmust be properly controlled to obtain a good etching selectivity.

FIG. 4 shows an etching apparatus 30 according to the prior art. Asshown in FIG. 4, the etching apparatus 30 comprises a processing tank32, a pre-heating tank 34 and an etching solution consisting ofphosphoric acid and deionized water. During the etching process, theetching solution in processing tank 32 is heated and maintained at 150°C.˜160° C. to remove the silicon nitride layer 16 from the wafer 10.Phosphoric acid from the facility is pre-heated to 120° C.˜140° C. inthe pre-heating tank 34 and then transferred to the processing tank 32via the pipe 36 to supply the etching solution discharged via thepipeline 38.

FIG. 5 and FIG. 6 show the variation of the silicon concentration of theetching solution in the processing tank 32. As shown in FIG. 5,silicon-containing impurity is generated during the etching reaction ofthe silicon nitride, and the silicon concentration of the etchingsolution in the processing tank 32 increases as the processing time ofthe etching reaction (i.e. reaction time) increases. When the siliconconcentration of the etching solution increases continually to asaturation state (about 100 ppm), silicon particles will be generated.The silicon particles will seriously influence the clearness of theetched surface of the wafer 10. For example, a 0.2 μm silicon particleremaining on the surface of wafer 10 will seriously cause integratedcircuit fabricated by a 0.131 μm MOS fabrication process to fail.

Referring to FIG. 4, in order to avoid the formation of the siliconparticles, the conventional etching apparatus 30 circulates and filtratethe etching solution continually in the processing tank 32 by the pipe42 and the filter 44 to remove silicon particles therein. However, ifthere were too many silicon particles, the filter 44 would easily faildue to the blocking of the silicon particles. Therefore, after theetching reaction is performed for certain number of times (i.e. beforethe silicon concentration reaches 100 ppm), the etching solution in theprocessing tank 32 must be entirely dumped via the pipe 38, and acompletely new etching solution (the silicon concentration is zero) issupplied into the processing tank 32 via the pre-heating tank 34 toprevent the formation of silicon particles due to the silicon saturationof etching solution. Consequently, the variation curve 52 of the siliconconcentration of the etching solution in the processing tank 32 presentsa zigzag curve between 0 and 100 ppm, as shown in FIG. 5.

The etching selectivity between the silicon nitride and the siliconoxide primarily depends on the silicon concentration of the etchingsolution. However, the silicon concentration of the etching solution inthe processing tank 32 does not maintain at a fixed level, but changesfrom zero (when a new etching solution is refilled in the processingtank 32) to silicon saturation concentration gradually. Therefore, theetching selectivity between silicon nitride and silicon oxide alsochanges with the processing time of the etching solution, which furtherincreases the difficulty to control the process parameters, such as theetching time.

According to the treating method currently used in semiconductorfabrication, dummy wafers are used to carry out several dummy runs asthe etching solution is renewed entirely (silicon concentration is zero)to increase the silicon concentration of the etching solution to apredetermined level, and the practical etching process of the actualwafer is carried out. However, this treating method obviously reducesthe efficiency of the etching solution. Furthermore, completely renewingthe phosphoric acid etching solution increases the consumption ofphosphoric acid and raises the etching cost.

Please refer to FIG. 6, wherein another conventional method for treatingthe etching solution periodically drains a portion of the phosphoricacid etching solution via the pipe 38, and supplies an equal amount ofnew phosphoric acid to the processing tank 32 via the pipe 36. As aresult, the variation curve of the silicon concentration of the etchingsolution in the processing tank 32 has a smaller variation range.Compared with the silicon concentration in the processing tank 32 whichchanges with the processing time of the etching reaction, phosphoricacid in the pre-heating tank 34 is directly supplied from the facilitypipe 140 and the silicon concentration is virtually zero since there isno resource for generating silicon. Therefore, when the etching solutionin the processing tank 32 is entirely renewed according to this treatingmethod, it should carry out several dummy runs using the dummy wafers toincrease the silicon concentration of the etching solution.

SUMMARY

The objective of the present invention is to provide an etching systemand a method for treating the etching solution with a stable selectivitybetween silicon nitride and silicon oxide.

In order to achieve the above-mentioned objective and avoid the problemsof the prior art, the present invention provides an etching system and amethod for treating the etching solution with a stable selectivitybetween silicon nitride and silicon oxide. The present etching systemcomprises a processing tank with an etching solution containing silicon,a cooling tank, a pre-heating tank, a first pipe for transferring theetching solution from the processing tank to the cooling tank, a secondpipe for transferring the etching solution from the cooling tank to thepre-heating tank, and a third pipe for transferring the etching solutionfrom the pre-heating tank to the processing tank.

The present method for treating the etching solution first performs anetching process using the etching solution, which is then cooled to afirst temperature to form a silicon-saturated etching solution. Aftersilicon-containing particles in the silicon-saturated etching solutionlarger than a predetermined size are filtered out, the silicon-saturatedetching solution is heated to a second temperature to form anon-saturated etching solution for performing another etching processlater. The second temperature is preferably at least 10° C. higher thanthe first temperature.

Compared with the prior art, the present invention possesses a steadier,smaller variation of the silicon concentration in the etching solution,and achieves a stable etching selectivity between the silicon nitrideand silicon oxide. In addition, the present invention need not drain theused etching solution, which can reduce the cost of the etching processdramatically.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objectives and advantages of the present invention will becomeapparent upon reading the following description and upon reference tothe accompanying drawings in which:

FIG. 1 to FIG. 3 show a method for fabricating a shallow trenchisolation on a wafer according to the prior art;

FIG. 4 shows an etching apparatus according to the prior art;

FIG. 5 and FIG. 6 show the variation of the silicon concentration of theetching solution in the processing tank;

FIG. 7 shows the relation of the silicon concentration in the etchingsolution with respect to both the etching rate and silicon particleconcentration;

FIG. 8 shows the relation between the silicon saturation concentrationof the etching solution and the temperature;

FIG. 9 illustrates an etching system according to the present invention;and

FIG. 10 shows the variation of the silicon concentration of the etchingsolution in the processing tank.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 7 shows the relation between the silicon concentration and both theetching rate and silicon particle concentration in the etching solution.Curve 72 represents the etching curve of silicon nitride, curve 74represents the etching curve of silicon oxide, and curve 76 is avariation curve of the silicon particle concentration. As shown in FIG.7, the etching rate of silicon nitride is substantially not influencedby the silicon concentration virtually and is fixed at about 90 Å/min.In contrary, the etching rate of silicon oxide reduces as the siliconconcentration increases, and is fixed at about 0.2 Å/min as the siliconconcentration is above 100 ppm. When the silicon concentration is above100 ppm, the silicon particle concentration of the etching solutionincreases as the silicon concentration increases.

FIG. 8 shows the relation between the silicon saturation concentrationof the etching solution (i.e. the solubility of silicon in the etchingsolution) and the temperature. As shown in FIG. 8, the siliconsaturation concentration is about 20 ppm at 10° C., 40 ppm at 120° C.,and 120 ppm at 160° C. That is, increasing the temperature of theetching solution will increase the solubility of silicon in the etchingsolution. In contrary, decreasing the temperature of the etchingsolution can force silicon in the etching solution to form siliconparticles (solid phase) and reduce the silicon concentration of theetching solution (liquid phase), wherein silicon particles in solidphase can be filtrated via a filter and removed from the etchingsolution.

FIG. 9 illustrates an etching system 100 according to the presentinvention. As shown in FIG. 9, the etching system 100 comprises aprocessing tank 102 with an etching solution containing silicon, acooling tank 104, a pre-heating tank 106, a pipe 112 for transferringthe etching solution from the processing tank 102 to the cooling tank104, a pipe 114 for transferring the etching solution from the coolingtank 104 to the pre-heating tank 106, and a pipe 116 for transferringthe etching solution from the pre-heating tank 106 to the processingtank 102. In addition, the pre-heating tank 106 can derive a new etchingsolution from a facility pipe 118.

The etching solution is cooled to a first temperature in the coolingtank 104, and the silicon concentration of the etching solution issaturated at the first temperature, wherein the first temperature ispreferably between 80° C. and 120° C. etching solutions then heated to asecond temperature in the pre-heating tank 106, and the siliconconcentration of the etching solution is not saturated at the secondtemperature, wherein the second temperature is preferably at least 10°C. higher than the first temperature. The etching solution from thecooling tank 104 is heated in the pre-heating tank 106, and thentransferred to the processing tank 102 via the pipe 116 to carry out awet etching process. The temperature of the etching solution in theprocessing tank 102 can be between 130° C. and 160° C. Preferably, theetching solution is heated in the pre-heating tank 106 directly to thetemperature at which the etching reaction is to be carried out, and thentransferred to processing tank 102 via the pipe 116.

The present etching system 100 can further comprise a filter 120 with aninlet 122 and an outlet 124, a pipe 132 for transferring the etchingsolution from the bottom of the cooling tank 10 to the inlet 122, and apipe 134 for transferring the etching solution from the outlet 124 tothe cooling tank 104. The filter 120 has a plurality of openings with asize smaller than 0.1 μm. The cooling tank 104 forces silicon in theetching solution to form solid silicon particles by reducing thetemperature of the etching solution. The solid silicon particle largerthan 0.1 μm will be filtered from the etching solution since it cannotpass through the openings of the filter 120 as the etching solution ispassing through the filter 120 in a downstream manner.

The present etching system 100 can further comprise a pipe 142 connectedto the inlet 122 and a pipe 144 connected to the outlet 124. Since theopenings of the filter 120 might be blocked by the silicon particles,the blocked silicon particles must be cleaned and removed frequently tomaintain the filtration function of the filter 120. According to thepresent invention, a solution containing hydrofluoric acid (for example,a diluted hydrofluoric acid) can be transferred in a downstream mannerfrom the pipe 142 to the filter 120 to dissolve the silicon particles onthe filter 120, and the dissolved silicon particles can then bedelivered out of the filter 120 from the pipe 144. The hydrofluoric acidremained on the filter 120 is then washed by deionized water. Inaddition, deionized water can be input via the pipe 144 to clean andremove the silicon particles on the filter 120 in an upstream manner,and waste liquid is discarded out of the filter 120 from the pipe 142.

The valves 131, 133 are closed on cleaning the filter 120 to preventsilicon particles on the filter 120 from flowing back to the coolingtank 104. When the filter 120 is filtrating silicon particles in thecooling tank 104, the valves 141, 143 are close. Furthermore, the valve113 can be closed during the cleaning of the filter 120 to temporarilystop supplying etching solution to the pre-heating tank 106. Since thepre-heating tank 106 stores some etching solution, the etching solutioncan be continuously supplied to the processing tank 102 during thecleaning of the filter 120. After the filter 120 is cleaned and thesilicon particles in the cooling tank 104 are filtrated, the valve 113is opened to supply the etching solution to the pre-heating tank 106.

As the design rule of the semiconductor fabrication shrinks, the allowedparticle size in etching solution decreases correspondingly. The filter120 with smaller openings must be used (for example, an opening smallerthan 0.1 μm). However, a smaller opening could easily fail due to theblocking of particles, and thus the filter 120 must be cleaned orreplaced more frequently to ensure the filtrating and removing of theparticles from the etching solution. During cleaning or replacing of thefilter 120, the pre-heating tank 106 can also supply continuouslyfilter-treated etching solution to the processing tank 102 according tothe present invention. In other word, the present invention can increasecleaning frequency of the filter 120 without influencing the supply ofthe etching solution. Consequently, the filter 120 with smaller openingscan be used in the future semiconductor fabrication process.

FIG. 10 shows the variation of the silicon concentration of the etchingsolution in the processing tank 102. The present invention controls thetemperature of the cooling tank 104 to indirectly control the siliconconcentration of the etching solution in the processing tank 102. Thesilicon concentration of the etching solution in the cooling tank 104 issaturated, and the saturated concentration is determined by thetemperature of the cooling tank 104. Without a new etching solutiondelivered into the pre-heating tank 106 from the facility pipe 118, thepre-heating tank 106 only heats the etching solution from the coolingtank 104 and the silicon concentration of the etching solution is notchanged. The silicon concentration of the etching solution from thepre-heating tank 106 to the processing tank 102 is maintained at apredetermined level, rather than zero silicon concentration.

Compared with the etching solution with a zero silicon concentrationadded into the processing tank 32, thus causing larger concentrationvariation according to the prior art (as shown in the curve 62 in FIG.10), the silicon concentration of the etching solution added intoprocessing tank 102 is not zero and the variation curve 92 of thesilicon concentration has smaller concentration variation according tothe present invention. For the present etching system 100, the etchingsolution can even be input into or output from the processing tank 102in a successive manner with a predetermined flow rate, and the variationcurve of the silicon concentration becomes steadier and smaller than thesaturation concentration.

Furthermore, compared with the prior art with a zero siliconconcentration of etching solution in the pre-heating tank 34 (see FIGS.4, 5 and 6), where the etching solution in the processing tank 32 mustbe periodically exchanged, the present invention does not need to carryout the dummy runs in the processing tank 102 since the siliconconcentration of the etching solution in the pre-heating tank 106 is notzero. The cooling tank 104 can supply the recycled etching solution tothe pre-heating tank 106; therefore the silicon concentration of theetching solution in the pre-heating tank 106 is not zero.

In addition, phosphoric acid is only used as a catalyst in the etchingsolution, which does not be consumed in theory when the etching reactioncarries on. However, the used etching solution must be drained, whichwill increase the cost of the etching process and raise the additionalcost on treating etching waste liquid according to the prior art. Incontrary, the present invention need not drain the used etchingsolution, which can reduce the cost of the etching process dramatically.

Briefly, the present method for treating the etching solution uses anetching solution to perform an etching process to a silicon-containingfilm, and the etching solution is then cooled to 80° C.˜120° C. to forman etching solution with a silicon concentration at a saturated state.After silicon particles larger than a predetermined size (for example,0.1 μm) are filtrated and removed from the saturated etching solution,the saturated etching solution is heated up at least 10° C. to form anon-saturated etching solution, which is then used to perform anotheretching process.

The above-described embodiments of the present invention are intended tobe illustrative only. Numerous alternative embodiments may be devised bythose skilled in the art without departing from the scope of thefollowing claims.

1. Method for treating an etching solution, comprising steps of:performing an etching process for stripping a silicon nitride film in aprocessing tank by using an etching solution including phosphoric acid;cooling the etching solution to a first temperature such that thesilicon concentration of the etching solution is saturated at the firsttemperature to form a silicon-saturated etching solution and siliconparticles; removing the silicon particles; heating the silicon-saturatedetching solution to a second temperature to form a non-saturated etchingsolution; and implanting the non-saturated etching solution to theprocessing tank; wherein the silicon concentration of the etchingsolution is maintained at a predetermined level other than a zerosilicon concentration.
 2. The method for treating an etching solution ofclaim 1, wherein the first temperature is between 80° C. and 120° C. 3.The method for treating an etching solution of claim 1, wherein thesecond temperature is at least 10° C. higher than the first temperature.4. The method for treating an etching solution of claim 1, wherein thestep of removing the silicon particles is performed by filtering siliconparticles larger than a predetermined size from the reuse etchingsolution in a subsidiary loop.
 5. The method for treating an etchingsolution of claim 4, wherein the step of filtrating silicon particles isperformed by passing the reuse etching solution in a downstream mannerthrough the filter with a plurality of openings smaller than 0.1 μm toremove silicon particles larger than the openings from the etchingsolution.
 6. The method for treating an etching solution of claim 1,wherein the etching process is performed at the second temperature. 7.The method for treating an etching solution of claim 1, wherein thenon-saturated etching solution further comprises a fresh etchingsolution.